How would you design a notification or chat system (fan-out, presence)?

Question

Accepted Answer

## Step 1: Use Cases, Constraints, Assumptions, Back-of-Envelope

### Use cases
We'll scope to handle only the core flows that expose the bottlenecks; everything else is deferred.

- **User** sends message/notification — POST to API, persisted, fanned out to online recipients via WebSocket, offline via push (APNs/FCM).
- **User** receives — subscribed via WebSocket/SSE, presence shows online/offline/typing.
- **Service** fans out — group chat fan-out O(members), notification fan-out via queue + pub/sub.
- **Service** stores history — paginated, searchable, retention 1y, E2E optional.

### Out of scope
- E2E encryption key management, spam filtering ML, message editing deep history, voice/video (separate SFU).

### State assumptions
- 10M DAU chat, avg 20 messages/user/day → 200M messages/day, avg 200 bytes → 40 GB/day.
- Concurrent connections 1M (10% DAU), each WebSocket 10 KB server memory → 10 GB for gateways.
- Fan-out: 1:1 is 1 write, group 100 members → 100 fan-out writes; notifi

How would you design a notification or chat system (fan-out, presence)?

The problem

Try it yourself

Solution

Step 1: Use Cases, Constraints, Assumptions, Back-of-Envelope

Use cases

Out of scope

State assumptions

The solution is hidden so you can try first

Discussion

Keep your momentum going.